A basic requirement in the design of a permanent magnet is the efficient use of the energy stored within the magnetized material. In general, the design should be aimed at achieving the desired value of the intensity within the cavity of the magnet with a minimum volume and weight of the magnetized material.
Permanent magnets can be classified as yokeless magnets where the magnetic structure is designed in such a way that the field in confined within the magnet without the need of a high magnetic permeability yoke; yoked magnets where the magnetic material is used to generate the desired field within the region of interest and the field confinement is achieved with an external yoke, or hybrid magnets wherein the magnetic structure is designed to require only a partial yoke.
Yokeless permanent magnets can be designed with magnetic materials which have a quasi-linear demagnetization characteristic with a slope close to that of air. In this case, the magnetic structures are transparent to the field generated by other sources. This is an important property of the yokeless structures, which allows the designer to increase the strength of the field within the cavity by superimposing the fields generated by concentric magnets.
Yoked permanent magnets, on the other hand, are shielded from external sources by the same yoke which confines the field of the magnet. As a consequence, the field superposition property of yokeless structure does not apply to yoked magnets and the field strength attainable within the cavity has an upper limit dictated by the magnet geometry. Conversely, because the magnetic material performs only the function of generating the field within the cavity, in general yoked structures use less magnetic material than yokeless structures designed for the same field strength and the same geometry of the cavity. Thus, weight of the magnetic material may become an important factor in the choice between a yokeless and a yoked magnet design, particularly if the design parameters dictate the use of high energy, high cost magnetic materials.
The design methodology for two dimensional as well as three dimensional yoked and yokeless magnetic structures capable of generating a uniform field within a cavity of arbitrary geometry, and design optimization of such geometries, are discussed, for example, in "Linear Theory of Yokeless Permanent Magnets, M. G. Abele, EMMA '89, Rimini, Italy, 1989; "Design of Yokeless Rare Earth Magnets for NMRMedical Applications". M. G. Abele, Proceedings of the 10th International Workshop on Rare Earth Magnets, Kyoto, Japan, 1989, pp121-130; and "Optimum Design of Two-Dimensional Permanent Magnets", M. G. Abele, TR-21, Department of Radiology, New York University, 1989. This subject is also discussed in U.S. patent application Ser. No. 424,162 filed Oct. 19, 1989 and U.S. patent application Ser. No. 707,620 filed May 30, 1991 in the name of M. G. Abele, and assigned to the assignee of the present application. The above publications and applications are incorporated herein by reference.